ShellPkg: Fix a bug ">>v" cannot append data to environment variable

ShellPkg: Fix a bug ">>v" cannot append data to environment variable

When ">v" is used to redirect the command output to environment
variable, the ending "\r\n\0" is removed before setting to environment
variable but the length is not updated.
It causes ">>v" fails to append data to the environment variable
created by ">v".
The patch fixes the above bug.

Signed-off-by: Chen A Chen <chen.a.chen@intel.com>
Reviewed-by: Ruiyu Ni <ruiyu.ni@intel.com>
Reviewed-by: Jaben Carsey <jaben.carsey@intel.com>